Multicarrier modulation (MCM), especially OFDM, is used in a wide variety of wireless digital communication applications, including Digital Audio Broadcasting (DAB), wireless Local Area Networks (LAN), and Terrestrial Digital Video Broadcasting (DVBT). In an OFDM system, data is carried on narrowband sub-carriers in the frequency domain. The data is transformed into the time domain for transmission (e.g., using an inverse fast Fourier transform (IFFT)) and then transformed back to the frequency domain at the receiver (e.g., using an FFT). The popularity of OFDM is due in part to its robustness against multipath delay spread and narrowband interference, high data transmission rate capability, and spectral efficiency.
Coherent modulation using quadrature amplitude modulation (QAM) instead of differential modulation schemes such as differential phase shift keying (DPSK) is preferred for OFDM applications due to its improved signal-to-noise ratio (SNR) performance. QAM, however, requires channel estimation to determine best possible decision boundaries for the constellation of each sub-carrier. Hence, channel estimation is a crucial part of OFDM to achieve high data rate performance.
Techniques for tracking symbol timing can be categorized as pre-FFT and post-FFT. One example of a post-FFT technique derives tracking information from the relative phases of the demodulated known pilots, utilizing the fact that the received phase of a pilot equals the product of its carrier frequency and the multipath delay. Thus, the slope of the phase profile as a function of carrier frequency defines the delay. However, this approach does not resolve the individual multipath components so will tend to be dominated by the strongest component; also, the phase has to be “unwrapped” to allow for cycle ambiguity. An example of a pre-FFT technique exploits the repetitive waveform of the cyclic prefix by computing the autocorrelation, defined as the product of the received waveform sample and the complex conjugate of the waveform sample received earlier by TU (where TU is the FFT processing duration and the OFDM data duration as shown in FIG. 1). This autocorrelation has an average value of zero except when there is a match between the cyclic prefix and the waveform repetition at the end of the OFDM data. The problem solved by the present invention is to determine how to set the FFT timing in the receiver near the leading edge of the portion of the nonzero autocorrelation due to the earliest multipath component.
Many solutions for the multipath timing synchronization have been proposed. For example, N. Chen et al., propose “OFDM Timing Synchronization Under Multi-path Channels”, IEEE 2003. The schemes described in this reference, however, require large amounts of averaging and, hence, long response times when operating at low SNRs. Another proposal by C. Williams et al, “Robust OFDM Timing Synchronization”, Electronics Letters June 2005, fails to synchronize if the difference in relative power between the first path (which one wants to track) and the later paths is more than 6 dB, with the second path being stronger. Donghoon Lee and Kyungwhoon Cheun, “Coarse Symbol Synchronization Algorithms for OFDM Systems in Multipath Channels”, IEEE Communication letters, October 2002 propose a method for coarse symbol acquisition but this scheme is not suitable for time tracking. Rohit Negi and John M Cioffi, “Blind OFDM Symbol Synchronization in ISI channels”, September 2002, IEEE Transactions on Communications present a computationally demanding method for channel estimation, requiring multiple singular value decompositions per symbol. As such it cannot be implemented with low complexity in an application specific integrated circuit (ASIC). Finally, the process proposed by Karthik Ramasubramanian and Kevin Baum, “An OFDM Timing Recovery Scheme with Inherent Delay-Spread Estimation”, IEEE GLOBECOM '01, vol 5, pp. 3111-3115 (2001) has a similar problem to that proposed by Williams et al., in that it cannot distinguish paths with low powers relative to larger secondary paths.